An electronic endoscope has normal and high-sensitivity image sensors. In a special mode for imaging an oxygen saturation level of blood, one of the normal and high-sensitivity image sensors is selected in accordance with a reflected light amount of special illumination light. When the normal image sensor is selected, the normal image sensor captures an image under irradiation with the special illumination light and outputs a second normal-sensor image. When an average pixel value of the second normal-sensor image is less than a first sensor selection threshold value, the use of the high-sensitivity image sensor is started. When the high-sensitivity image sensor is selected, the high-sensitivity image sensor captures an image under irradiation with the special illumination light and outputs a high-sensitivity-sensor image. When the average pixel value of the high-sensitivity-sensor image is more than a second sensor selection threshold value, the use of the high-sensitivity image sensor is stopped.